Another interestingly structured fossil stromatolite - replaced by a mix of paled blueish chalcedony, white crystallized quartz and brown limonite. This makes it an agate. The agate shows some black hematite in the matrix, and minor in the cut. Of course, it's cut and polished. Nice growth structures, if I interprete the specimen right.

This comes from the same geological structure as the other 'fossil stromatolites' and 'fossil fungi' inclusions in quarz crystals from the Sauerland area - "type locality" Warstein. The material is confined to karstic hollow molds of Devonian 'Massenkalk'. An Oligocene/Miocene age is presumed according to the development of the karst, the mineral paragenesis, and microthermometrical measurements. Fungal mycelia grew on cracks and other hollow molds. Percolating water transported organic C originating from the soil-/sedimentary cover as a substratum for the fungi as well as chelated and free Fe2+ and colloidal Fe3+. Iron was precipitated on the cell walls of the fungal filaments, and thus further precipitation was initiated. Iron-rich layers formed. Gradual dehydration finally led to the formation of goethite. Gel splits and settling structures have often been observed in these regions. The silicification starting within this phase conserved the partially rather fragile structures. The iron stromatolites of Warstein are probably the first examples of fossil biogenic, non-photosynthetic laminites.

New publications attribute the organic stromatolitic structures to fossil bacteria.

Picture colors may differ a little from the natural ones.